The present invention relates to portable electric tools capable of obtaining an overspeed condition and in particular to portable electric tools having electronic speed control circuits which regulate the speed of the tool at a substantially constant speed regardless of operating load.
Portable electric tools are often equipped with electronic speed control circuits which regulate the speed of rotation of the motor shaft and which, when working properly, maintain a constant speed regardless of the load placed on the tool. A malfunction of such an electronic speed control circuit can result in the motor of the electric tool obtaining an overspeed condition.
Devices for preventing the operation of a tool at excessively high rates of speed are known in the art. One such measure is shown in German Pat. No. 1,763,279 which relates to use of a centrifugally actuated knife to cut the electric lead supplying power to the tool motor. Other alternative devices are broadly suggested in British Pat. No. 1,271,431, which suggests devices to isolate the main power supply if the speed exceeds a maximum permissible value or alternatively to limit speeds to less than a maximum value by means of a speed limiting circuit. There remains, however, a need for an improved overspeed protective arrangement which protects against operation at an overspeed condition and yet also allows continued use of the tool thereafter. It would, of course, be desirable if such an overspeed protective arrangement could be provided in a practical and economical manner so as not to unduly increase the cost of manufacturing the tool.